High molecular weight, high density polyethylene (HMW HDPE) film resins for tough thin film applications are generally extruded by the bubble technique at high extrusion rates through small diameter dies. The high extrusion rates are conducted by the film manufacturer to minimize production costs so that products can be marketed at competitive prices. Generally small dies are used to attain the high blow up ratios required to produce film with sufficient strength for the various tough thin film applications. Because the performance requirements are very stringent only HMW HDPE resins, which have broad, bimodal molecular weight distributions (MWDs) are able to compete effectively in the marketplace. Resins that do have these characteristics do not extrude well at equally high rates because of bubble instability, higher variation of melt distribution within the die, excessive power usage and poorer film quality as compared to the leading resins. The poorer film quality can manifest itself as higher gauge variation, weaker strength and poorer appearance.
Although HMW HDPE resins produced by the UNIPOL Process possess numerous advantages, it has been found that they do not have the molecular structure required to produce film for the highest performance applications because of extrusion and film strength deficiencies. Studies to overcome these deficiencies have focused on changes in key resin properties (molecular weight, molecular weight distribution, (MWD) and comonomer) and the development of new additives, particularly those additives that enhance the extrusion characteristics of the resin, such as improved bubble stability, film gauge variation reduction and melt fracture elimination. Fluoroelastomers are effective in eliminating the melt fracture generally encountered with HMW HDPE and can slightly reduce power draw (5%) during extrusion by forming a lubricating coating on the extruder barrel, screw and die (see U.S. Pat. No. 3,125,547 issued on Mar. 17, 1964 and assigned to Dupont Corporation). However the fluoro elastomers require a long extrusion period -- approximately one hour -- to complete the coating. This creates higher amounts of scrap film for the film producer, which the film producer does not incur with leading competitive resins. Additionally, unacceptably high film gauge variation is frequently encountered because the fluoroelastomer coating is not always distributed uniformly. As a result, the higher gauge variation can reduce film strength and detract from the appearance of the film.
Zinc stearate and blends of zinc stearate and calcium stearate have also been demonstrated to eliminate melt fracture and reduce film gauge variation resulting from use of HMW HDPE resins without lengthy equipment conditioning periods. However, melt fracture can recur at high extrusion rates and/or low melt temperatures when used in high molecular weight HDPE film resin (&lt;7.0 HLMI) that do not have very broad molecular weight distributions (Mw/Mn &lt;20). Increasing the melt temperature will broaden the extrusion rate range over which melt fracture is not incurred with the stearates, but higher melt temperatures will also limit extrusion rates by lowering melt strength, which causes higher bubble instability. In addition to lowering the extrusion rate, higher melt temperatures and poor bubble stability will also have a deleterious effect on film quality.
Current literature teaches the use of acrylic polymers for reducing melt fracture occurrence during processing of polyvinyl chloride (See for example the publication entitled "Acrloid K-147" processing aid for polyvinyl chloride by Rohm & Haas Company, Industrial Chemicals Department).
In addition, British Patent Specification 1,511,683 published May 24, 1978 and assigned to Rohm & Haas Company, Inc. discloses the use of 1% to 25% by weight of acrylic polymers produced from a monomer system comprising at least 50% by weight of methyl methacrylate for improving the processability of polyethylenes having a molecular weight of over 600,000 (Mv).
In an effort to reduce melt fracture occurring during processing of HMW HDPE resins of molecular weight of about 250,000 to 420,000 and produced by the low pressure fluidized bed process as disclosed in e.q., U.S. Pat. No. 4,508,842 or 4,383,095 it was found that surprisingly, not only was melt fracture reduced to acceptable levels but advantageously bubble stability was improved dramatically, film gauge variations were reduced significantly and melt fracture was eliminated at relatively high extrusion rates utilizing acrylic polymers at significantly lower concentrations than would be expected.
Indeed, the concentration found useful according to the present invention results in significantly lower costs of operation and advantaqeously in view of the low concentration utilized i.e., about 0.15% to about 0.75%. The films can be used in polyethylene products designated for food contact packaging.